Such a pump device is used for, e.g., circulation of a metalworking fluid (a coolant) in a machine tool. In this case, a low-pressure pump having a relatively low discharge pressure and medium-pressure and high-pressure pumps each having a relatively high discharge pressure are prepared, chips produced during machining are removed by using the low-pressure pump, machinability of a tool for deep hole drilling or the like is improved by the medium-pressure and high-pressure pumps, and chips are removed during machining. That is, the pumps are used properly for different purposes.
As the low-pressure pump, a positive-displacement pump such as a centrifugal pump is mainly used. Such a pump is advantageous for a high capacity, and a suction filter comparable to, e.g., a strainer can suffice and is superior in maintenance. However, to obtain a high pressure, impellers must be installed in multiple stages, and a long pump portion requires a large installation space as a drawback of this pump.
As the medium-pressure and high-pressure pumps, a positive-displacement pump such as trochoidal pump is superior in efficiency.
However, to obtain a high capacity using the positive-displacement pump, a high-capacity rotor is required, and a main body is also large in size. Further, the pump may possibly fail to operate properly due to jamming by foreign substances, and therefore a filter is required on the intake side. As a result, a pipe fitting working takes long time, and a filter needs regular maintenance.
Therefore, as the medium-pressure and high-pressure pumps, a non-positive-displacement pump is often adopted except for cases that discharge must be carried out at a high pressure that is substantially impossible to carry out when the non-positive-displacement pump is used.
Here, if foreign substances are mixed in a coolant used for a machine tool, they can be a cause of scratches of a material during machining or abrasion or damage of the tool. Therefore, for example, a sludge removal device is installed in a tank, a strainer for protecting pump is provided, or a filter is disposed separately from a suction filter, and then a clean coolant is supplied.
In particular, in case of a coolant liquid supplied using a high-pressure pump, since an ejected liquid may have a high velocity in some cases, an influence of foreign substances in the coolant on a material being machined is also considerable. Therefore, the coolant liquid must be cleaned in a rigorous manner.
For example, to clean an operating fluid like a coolant, various kinds of technologies have been conventionally suggested.
According to the prior art concerning a coolant pump equipped with a general type filter, the filter is disposed on the intake side of a positive-displacement pump driven by an electric motor. However, since a filter element is used for filtration, the filter element must be periodically cleaned or replaced.
Moreover, since the pump is heavy in weight because of its structure, it must be installed on a firm large platform that can bear the heavy weight. Additionally, since this prior art is large in size, it has a problem that installation is impossible unless a large installation space is present.
As another prior art, for example, there has been suggested a construction in which an impeller that rotates by an electric motor is connected to the intake side or the outflow side of a cyclone filter and a pump is arranged on the outflow side of this impeller (refer to Patent Literature 1).
However, since foreign substances stay in the cyclone filter according to this prior art, an operation for periodically removing the foreign substances which staying in the cyclone filter is required. Further, this prior art has a drawback that the impeller does not have self-priming properties and a liquid cannot be discharged until the impeller gets wetted.
Alternatively, a construction in which a centrifugal pump is provided on the outflow side of a cyclone filter has been also suggested (refer to Patent Literature 2). Furthermore, a construction in which an axial flow pump is provided on the outflow side of a cyclone filter has been also suggested (refer to Patent Literature 3).
However, these the prior arts (Patent Literature 2, Patent Literature 3) likewise require an operation for removing foreign substances staying in the cyclone filter. Moreover, since self-priming properties are not provided, a device constructed to suck (bring in) an operating fluid is additionally required on the intake side.